As technologies evolve, complementary metal-oxide semiconductor (CMOS) image sensors are gaining in popularity over traditional charged-coupled devices (CCDs) due to certain advantages inherent in the CMOS image sensors. In particular, a CMOS image sensor may have a high image acquisition rate, a lower operating voltage, lower power consumption and higher noise immunity. In addition, CMOS image sensors may be fabricated on the same high volume wafer processing lines as logic and memory devices. As a result, a CMOS image chip may comprise both image sensors and all the necessary logics such as amplifiers, A/D converters and the like.
CMOS image sensors are pixelated metal oxide semiconductors. A CMOS image sensor typically comprises an array of light sensitive picture elements (pixels), each of which may include transistors (switching transistor and reset transistor), capacitors, and a photo-sensitive element (e.g., a photo-diode). A CMOS image sensor utilizes light-sensitive CMOS circuitry to convert photons into electrons. The light-sensitive CMOS circuitry typically comprises a photo-diode formed in a silicon substrate. As the photo-diode is exposed to light, an electrical charge is induced in the photo-diode. Each pixel may generate electrons proportional to the amount of light that falls on the pixel when light is incident on the pixel from a subject scene. Furthermore, the electrons are converted into a voltage signal in the pixel and further transformed into a digital signal by means of an A/D converter. A plurality of periphery circuits may receive the digital signals and process them to display an image of the subject scene.
A CMOS image sensor may comprise a plurality of additional layers such as dielectric layers and interconnect layers formed on top of the substrate, wherein the interconnect layers are used to couple the photo diode with peripheral circuitry. The side having additional layers of the CMOS image sensor is commonly referred to as a front side, while the side having the substrate is referred to as a backside. Depending on the light path difference, CMOS image sensors can be further divided into two major categories, namely front-side illuminated (FSI) image sensors and back-side illuminated (BSI) image sensors.
In a FSI image sensor, light from the subject scene is incident on the front side of the CMOS image sensor, passes through dielectric layers and interconnect layers, and finally falls on the photo diode. The additional layers in the light path may limit the amount of light absorbed by the photo diode so as to reduce quantum efficiency. In contrast, in a BSI image sensor, light is incident on the backside of the CMOS image sensor without the obstructions from the additional layers. As a result, light can hit the photo diode through a direct path. Such a direct path helps to increase the number of photons converted into electrons.
In order to improve quantum efficiency of BSI image sensors, the substrate of BSI image sensors is thinned to a thickness in a range from about 2 um to 2.5 um. In addition, through an ion implantation process, a thin P+ layer having a thickness of about 100 Å may be formed on the thinned substrate to further improve quantum efficiency. Subsequently, a laser annealing process may be performed to activate the implanted P+ ions as well as repair crystal defects caused by the ion implantation process. Such a laser annealing process may cause dark mode image stripe patterns due to laser scanning boundary effects on the image sensor arrays.
Corresponding numerals and symbols in the different figures generally refer to corresponding parts unless otherwise indicated. The figures are drawn to clearly illustrate the relevant aspects of the various embodiments and are not necessarily drawn to scale.